When a patient is provided with a pacemaker, the pacemaker may be designed to generate heart stimulating pulses continually, or only when the patient's natural heart rate falls below a predetermined rate or (internal) threshold. In either case, the generated pulses will occur at a predetermined rate (the "pace rate.") In addition, some pacemakers are "rate responsive," which means that they automatically adjust the pace rate if the patient's suspected physical activity increases or decreases. They are many different systems used in pacemakers to predict when a patient's physical activity has increased, and therefore, when the pacing rate should be increased.
Several systems for varying the pacing rate of a pacemaker work on the assumption that increased physical motion means that the patient is engaging in physical activity, and that the pacing rate must therefore be increased. For example, the Elite.TM. Models 7074, 7075, 7076, and 7077 manufactured by Medtronic, Inc. of Minneapolis, Minn., may be programmed to vary the pacing rate in response to detected changes in body motion. A sensor within the device, typically a piezoelectric crystal placed on the inside wall of the pacemaker, detects pressure waves within the body caused by body motion. The device then converts these pressure waves into electrical signals. The pacing rate is set in proportion to the frequency and amplitude of these electrical signals. Other systems, such as the Relay.TM. Models 293-03 and 294-03 manufactured by Intermedics, Inc. of Freeport, Tex., use an accelerometer instead of a piezoelectric crystal to detect physical motion. The accelerometer computes acceleration by measuring the force exerted by restraints that hold a mass in a fixed position. The accelerometer may either be electrically excited or self-generating, using a piezoelectric crystal as discussed above. Some systems also include multiple accelerometers oriented in different axes, so that movement in different directions can be discerned and used to reduce to obtain are more accurate indication of the extent of physical movement.
However, the theory behind motion sensitive rate-responsive systems is fundamentally flawed because physical motion does not necessarily mean that a patient's physical activity has increased. For example, motion-based responsive systems will increase the pacing rate when a patient is driving down a bumpy road, even though there is no increase in physical activity. Likewise, a patient may undertake significant physical activity that does not involve movement of the motion sensor, such as when performing bench presses with heavy barbells. In such circumstances, no physical motion will be detected so the pacemaker will not increase the pacing rate, even though an increase would be appropriate. In short, motion-based responsive pacemakers can not distinguish between motions that relate to increased physical activity and those that do not. Furthermore, if unnecessary pacing is activated by these devices, battery power consumption results in a shortened battery life. Shortened battery life may require a patient to undergo a surgical procedure to replace the battery sooner than may otherwise be required. It is also desirable to develop a rate-responsive system that requires little power to operate so as to extend pacemaker battery life.
Due to the shortcomings of motion-based rate responsive sensors, other rate adjusting systems are responsive to certain physiological conditions of a patient. Some pacemaker systems vary the pace rate based on changes in body temperature. These systems use a temperature sensing device such as a thermistor to sample the temperature of the body. The thermistor is built into an electrode of the pacemaker. The resistance of the thermistor varies as a function of temperature so that the device can generate an electrical signal that corresponds to the sensed temperature. This signal may be translated into a pre-programmed activity level used to set the pace rate.
However, body temperature sensitive systems also result in many of the problems that occur in motion sensitive rate-responsive systems as changes in body temperature may occur without regard to physical activity. Thus, the system may attempt to filter out such extraneous temperature changes. Yet, such a filtering system poses the risk that changes in temperature that should be used to vary the pacing rate will be ignored. Further, because the thermistor is built into a specialized electrode, that electrode can only be used with a pacemaker sold by a particular manufacture, thereby limiting the physician's and patient's choice of pacemakers. Due to cost and insurance regulations, it is not normally feasible to replace a previously implanted electrode or pacemaker. Thus, temperature responsive systems have the shortcomings of being poor predictors of a change in physiological activity, and of having only a limited choice of pacemakers from which to select.
Other systems, referred to as QT systems, adjust the pace rate by determining the activity level as measured by the QT interval measurement; that is, the time between when a pacing pulse is sent to the heart and the time the QT interval of the heartbeat begins. Generally, as a patient's physical activity increases, the heart responds more quickly to a pacing pulse. Therefore, QT systems increase the pacing rate when they sense that this time period is reduced. A particular shortcoming of QT systems is that in order to sense when pacing should be initiated or increased, they must actually send a pacing pulse to the heart, regardless of the patient's intrinsic heart rate. In general, it is medically undesirable to send pacing pulses to the heart unless it is known that the heart actually requires a pulse. In addition, unnecessary pulses also needlessly consume battery power.
Another type of rate-responsive system is based on measuring a patient's blood oxygen saturation levels. These systems assume that when a patient increases physical activity, there is a corresponding increase in the blood oxygen saturations level. These systems employ special electrodes equipped with a light emitting diode ("LED") and a phototransistor which measures the occlusion or blockage between the two. This blockage roughly corresponds to the amount of oxygen in the blood tissue. While the device is based on a physiological phenomenon, it has two drawbacks. First, the LED require a significant amount of power which reduces the life of the battery powering the pacemaker. Additionally, the system requires special electrodes that may lock the physician into the selection of a particular system.
Rate-responsive systems are also available which depend upon a patient's respiration to alter the pacing rate. These systems assume that increased air volume in the lungs means a patient is breathing deeper, suggesting that the patient is engaged in increased physical activity. Alt et al., "Function and Selection of Sensors for Optimum Rate-Modulated Pacing," New Perspectives In Cardiac Pacing, ed. Barold et al., 1991, p. 189-196. Specifically, such a system measures the respiratory rate by detecting the electrical impedance between an auxiliary electrode lead and the pacemaker can. Because the electrical conductivity of lung tissue decreases with inspiration, breathing can be detected by monitoring changes in electrical resistance. The resistance can be measured between different points within the system. One can apply a current between the pacemaker can and an anodal ring of a bipolar electrode so that the system measures the resistance change between the pacemaker can and the tip of the bipolar electrode.
However, one disadvantage of these respiration systems is their sensitivity to movement. For example, a respiration-dependent system is likely to detect high impedance changes if the pacemaker user moves his arms or has chest movements. Such measurements may be interpreted by the system as deep breaths that require an increased pace rate. Further, these system requires bipolar electrodes as one wire is needed to emit an RF wave and another wire is required to sense voltage.
Yet another type of rate-responsive pacemaker is based on the determination of changes in the stroke volume of the heart. Alt et al., p. 172-177. As an individual increases physical activity, the stroke volume of the heart increases, regardless of whether the heart rate remains constant due to a condition such as chronotropic incompetence. To measure stroke volume a specialized electrode having two or more electrode poles is positioned with the right ventricle. These impedance-based rate-responsive systems transmit a low-amplitude AC pulse or short intermittent electrical pulses to the multipolar electrode to measure resistance between the electrode poles. As resistance is affected by the amount of blood between the electrode poles, stroke volume is estimated. The pacing rate is increased when an increase in heart stroke volume is detected. However, these systems also have limitations. First, only a portion of the stroke volume may be measured as the measurements are dependent upon the position of the electrode pairs. Also, a multipolar rather than a unipolar electrode is required and the electrode must be carefully placed into position. Furthermore, stroke volume may only be measured for endocardial lead systems, i.e., those residing within the heart, but not for epicardial lead systems. Therefore, selection of a pacemaker that uses a specialized electrode is limited. Though stroke volume is a physiological phenomenon which accurately reflects the pacing requirements, it is desirable to provide a rate-responsive system using stroke volume which is not dependent upon the critical placement of multipolar specialized electrode. It is also desirable to provide a rate-responsive system which works in conjunction with both endocardial and epicardial lead systems.